The present invention relates generally to reverse osmosis membranes and more particularly to a novel composite polyamide reverse osmosis membrane and to a method of producing the same.
It is known that dissolved substances can be separated from their solvents by the use of various types of selective membranes, examples of such membranes including microfiltration membranes, ultrafiltration membranes and reverse osmosis membranes. One use to which reverse osmosis membranes have previously been put is in the desalination of brackish water or seawater to provide large volumes of relatively non-salty water suitable for industrial, agricultural or home use. What is involved in the desalination of brackish water or seawater using reverse osmosis membranes is literally a filtering out of salts and other dissolved ions or molecules from the salty water by forcing the salty water through a reverse osmosis membrane whereby purified water passes through the membrane while salts and other dissolved ions and molecules do not pass through the membrane. Osmotic pressure works against the reverse osmosis process, and the more concentrated the feed water, the greater the osmotic pressure which must be overcome.
A reverse osmosis membrane, in order to be commercially useful in desalinating brackish water or seawater on a large scale, must possess certain properties. One such property is that the membrane have a high salt rejection coefficient. In fact, for the desalinated water to be suitable for many commercial applications, the reverse osmosis membrane should have a salt rejection capability of at least about 97%. Another important property of a reverse osmosis membrane is that the membrane possess a high flux characteristic, i.e., the ability to pass a relatively large amount of water through the membrane at relatively low pressures. Typically, the flux for the membrane should be greater than 10 gallons/ft.sup.2 -day (gfd) at a pressure of 800 psi for seawater and should be greater than 15 gfd at a pressure of 220 psi for brackish water. More preferably, the flux for the membrane is at least about 22 gfd at brackish water conditions. For certain applications, a rejection rate that is less than that which would otherwise be desirable may be acceptable in exchange for higher flux and vice versa.
One common type of reverse osmosis membrane is a composite membrane comprising a porous support and a thin polyamide film formed on the porous support. Typically, the polyamide film is formed by an interfacial polymerization of a polyfunctional amine and a polyfunctional acyl halide.
An example of the aforementioned composite reverse osmosis membrane is disclosed in U.S. Pat. No. 4,277,344, inventor Cadotte, which issued Jul. 7, 1981, and which is incorporated herein by reference. The aforementioned patent describes an aromatic polyamide film which is the interfacial reaction product of an aromatic polyamine having at least two primary amines substituents with an aromatic acyl halide having at least three acyl halide substituents. In the preferred embodiment, a porous polysulfone support is coated with m-phenylenediamine in water. After removal of excess m-phenylenediamine solution from the coated support, the coated support is covered with a solution of trimesoyl chloride dissolved in "FREON" TF solvent (trichlorotrifluoroethane). The contact time for the interfacial reaction is 10 seconds, and the reaction is substantially complete in 1 second. The resulting polysulfone/polyamide composite is then air-dried.
Although the Cadotte membrane described above exhibits good flux and good salt rejection, various approaches have been taken to further improve the flux and salt rejection of composite polyamide reverse osmosis membranes. In addition, other approaches have been taken to improve the resistance of said membranes to chemical degradation and the like. Many of these approaches have involved the use of various types of additives to the solutions used in the interfacial polycondensation reaction.
For example, in U.S. Pat. No. 4,872,984, inventor Tomaschke, which issued Oct. 10, 1989, and which is incorporated herein by reference, there is disclosed an aromatic polyamide membrane formed by (a) coating a microporous support with an aqueous solution comprising (i) an essentially monomeric, aromatic, polyamine reactant having at least two amine functional groups and (ii) an amine salt to form a liquid layer on the microporous support, (b) contacting the liquid layer with an organic solvent solution of an essentially monomeric, aromatic, amine-reactive reactant comprising a polyfunctional acyl halide or mixture thereof, wherein the amine-reactive reactant has, on the average, at least about 2.2 acyl halide groups per reactant molecule, and (c) drying the product of step (b) so as to form a water permeable membrane.
The amine salt of Tomaschke is said to be monomeric (i.e., polymerizable) and is said preferably to be a water soluble salt of a strong acid and a tertiary amine selected from the group consisting of a trialkylamine, such as trimethylamine, triethylamine, tripropylamine; an N-alkylcycloaliphatic amine, such as 1-methylpiperidine; and N,N-dialkylamine, such as N,N-dimethylethylamine and N,N-diethylmethylamine; an N,N-dialkyl ethanolamine, such as N,N-dimethylethanolamine; a bicyclic tertiary amine, such as 3-quinuclidinol and mixtures thereof, or a quaternary amine selected from at least one member of the group consisting of a tetraalkylammonium hydroxide, such as tetramethylammonium hydroxide, tetraethylammonium hydroxide and tetrapropylammonium hydroxide; a benzyltrialkylammonium hydroxide, such as benzyltrimethylammonium hydroxide, benzyltriethylammonium hydroxide and benzyltripropylammonium hydroxide; and mixtures thereof.
The present inventors have noted that all of the amine salts disclosed by Tomaschke are the product of a strong acid and a monofunctional tertiary amine and that Tomaschke does not teach or suggest the use of a water soluble salt of a strong acid and a polyfunctional tertiary amine.
Other patents of interest include U.S. Pat. No. 5,614,099, inventors Hirose et al., which issued Mar. 25, 1997, U.S. Pat. No. 5,576,057, inventors Hirose et al., which issued Nov. 19, 1996, U.S. Pat. No. 4,983,291, inventors Chau et al., which issued Jan. 8, 1991, U.S. Pat. No. 4,761,234, inventors Uemura et al., which issued Aug. 2, 1988, and U.S. Pat. No. 4,769,148, inventors Fibiger et al., which issued Sep. 6, 1988, all of which are incorporated herein by reference.